Paper, Order, or Assignment Requirements

Assessment
You are required to build a mouse-trap powered vehicle, from a kit of parts, within two 2 hour sessions. The frame, chassis and wheels can be constructed on SOLIDWORKS to produce laser cut-outs of MDF board. You will then take part in a race against all other team vehicles in a third 2 hour session.
This assignment carries 25% of the marks for this term i.e. 12.5% of the total module. The laboratory practical and the accompanying lab report is a group activity. The report should be no longer than six sides of A4, including diagrams and should describe the design and construction of the vehicle. You may refer to the theory of vehicles included in the lab script which should provide motivation for your particular design. You must also include the theoretical calculations (see later section) in your lab report, as an appendix.
Assessment Criteria
Clear, concise descriptions·
Justification of design based upon theory·
Correct calculations·
The aims of the exercise are:
to design and build a high-performance (high speed) vehicle to given· specifications based upon fundamental principles of mechanics;
to determine the most successful design from the results of a race· against other vehicle designs;
to calculate the performance of a theoretical vehicle and describe· differences that exist between
theory and practice.
MECH117 Lab 2 Draft Report Sheet – 1 –
The intended learning outcomes are that by the end of the exercise you should be able to:
Apply some fundamental principles of mechanics to a simple vehicle;·
Build and test a vehicle within the allocated time and with the· resources available;
Demonstrate effective team working skills.·
Mouse-Trap Vehicle
Description
A mouse-trap vehicle is one that is powered by the energy which can be stored in a wound up mouse-trap spring. The most basic design is as follows: a string is attached to a mouse-trap’s lever arm and then the string is wound around a drive axle causing the mouse-trap’s spring to be under tension. Once the mouse-trap’s arm is released, the tension of the mouse-trap’s arm pulls the string off the drive axle causing the drive axle and the wheels to rotate, propelling the vehicle.
Performance
The vehicle needs to be built for speed as it will take part in a race, but it must also be able to perform over a long enough distance to reach the finish line. The speed performance of the car will be affected by a number of parameters that you can address. These are:
1. Position of the mouse trap in relation to the drive axle. The closer that the mouse trap is to the drive axle the faster the vehicle will travel. If the drive wheels slip it is too close to the drive axle.
2. Maximise traction force between the wheels and the track by using rubber bands or section of a balloon.
3. Decrease the rotational inertia of the wheels. This can be done by removing mass from the inside of the wheels or using smaller diameter wheels.
4. Adjusting the wheel-to-axle diameter ratio by adding or removing tape on the drive axle.
5. Ensure that string alignment is attached directly over the drive axle when the lever arm is in the fully wound position.
There are five appendices that provide further information on the theoretical basis for the above design issues.
Theoretical Calculations
Table 1 contains results from an imaginary test to determine the spring constant and hence the stored energy of a mouse-trap spring.
The spring constant, k, is related to the spring lever angle (in radians) and the torque by
is the angle measured from its zero torque position (inq , where q k=T  radians).
MECH117 Lab 2 Draft Report Sheet – 2 –
For the data contained in Table 1 calculate the torsional spring constant, in units of Nm/rad.
The potential energy stored in the spring is given by P.E 2 ,q 1 k= qT dò =.
2
where θ is measured from zero torque position (measured in radians)
(a) Using the equations provided, calculate the stored (potential) energy of the mouse trap spring which has an initial extension of 170°.
Table 1: Imaginary test data
Angle of spring lever
(°)q
Measured torque T (Nm)
20
0.24
40
0.38
60
0.47
80
0.52
100
0.60
120
0.69
140
0.73
160
0.82
(b) Assuming that 85% of the potential energy is converted into kinetic energy and the vehicle has a mass of 150 grams, calculate the velocity of the vehicle when the lever angle reaches 0°. Also, what would be the theoretical time for the vehicle to travel 5 m (based on the assumption that lever angle reaches 0° at 5m and acceleration is constant).
Figure 1: Velocity Displacement Curve
s (m)
(c) Assuming the vehicle reaches the halfway distance of 2.5 m when the mousetrap is half closed (when θ=85o), calculate the velocity at the half way point. Calculate the constant acceleration and time for each half stage and compare the total time (to travel 5m) with question part (b) above.
(d) Finally, plot (or sketch) the vehicle velocity-displacement graph for part (a) and part (b) on the same plot, commenting on why it differs from the graph shown in Figure 1. Give full justification of your calculations, defining all variables used.